This invention relates to circulatory support devices. More particularly the invention relates to pulsatile circulatory support devices for use in pediatric patients.
The need exists for the use of a circulatory assist device in children and small infants critically ill with heart failure and unresponsive to maximal pharmacologic support. This clinical problem is common in patients following surgery for the correction of congenital heart defects and in patients suffering from acute myocarditis. In both cases, circulatory support aids in the patient's recovery. In patients suffering from irreversible heart failure, a circulatory assist device can act as a bridge to transplantation. A support device can provide for a means of keeping the pediatric patient alive while an acceptable heart donation is found. A device with the abilities to support a patient with terminal heart failure, while awaiting a donor's heart, is highly beneficial in order to maintain an overall organ perfusion and patient well-being thus improving the chance of transplant success.
Currently, centrifugal pumps or extracorporeal membrane oxygenation (hereinafter "ECMO") provide temporary circulatory support for pediatric patients. These methods of providing circulatory support for children have an operative mortality rate in the range of 5 to 25%, depending on the severity of the lesion, the age of the patient, and the expertise of the involved medico-surgical team. The use of centrifugal pumps and ECMO are limited by the higher complication rates caused by these systems. For example, these devices require: a large priming volume resulting in hemodilution; large surface areas leading to hypothermia and to activation of the complement system; high activated clotting time (hereinafter "ACT") levels causing bleeding complications; significant anticoagulants, that in turn cause subsequent bleeding complications; and these devices fail to supply a pulsatile flow. Moreover, these systems require a dedicated technician during the pump run, which may last for several days.
When using ECMO, the priming volume of the whole circuit represents up to three times the patient's blood volume. Although a centrifugal pump requires less priming volume, about 150 cc for the Biomedicus BP 50 "pediatric" head plus tubings, this is nearly one-half the blood volume of a newborn infant. The resulting dilution contributes to coagulation deficiencies and deleterious edematous effects on renal and cerebral functions. In addition, because infants up to six months of age have a poor thermoregulation system, lengthy tubing used in ECMO can promote heat loss leading to hypothermia and impaired coagulation mechanisms.
During ECMO use, the usual recommended level of anticoagulation is such that ACT is kept between 150 to over 200 seconds, depending on the particular system in use. In post-cardiotomy infants, often operated on under deep hypothermia, both dilutional thrombocytopenia and hypothermia induced platelet functional disorders are prominent. In addition, poor hepatic reserve and transient hepatic dysfunction from bypass reduce the production of clotting factors. During ECMO use, high ACT levels may lead to a high probability of bleeding complications, either mediastinal or cerebral.
It is believed that the failure to provide a pulsatile flow results in the high incidence of renal dysfunction during ECMO followed by recovery after the return to pulsatile flow. Moreover, a study in lambs by Champsaur et al., Pulsatility Improves Hemodynamics During Fetal Bypass, AHA, 66th Scientific Session, Atlanta, November 1993, paper 1797, p I-335, suggests that even in the short term, pulsatility improves hemodynamics during fetal bypass in which the systemic flow resistance is two times lower under pulsatile as compared to continuous bypass.
Prior pulsatile devices designed for pediatric use have been adaptations of adult devices. In an article by Yu et al., With and Without a Clamshell, TransAmerican Society Artificial Internal Organs, 1990, 36: M238-M242, the authors discuss the development of a 20 cc soft ventricular assist device (hereinafter "VAD") while requiring the additional development of a bileaflet valve to improve filling characteristics. Similarly, Taenaka et al., Experimental Evaluation and Clinical Application of a Pediatric Ventricular Assist Device, Trans American Society Artificial Internal Organs, 1989, 35: 606-608, has a scaled down 20 cc device using #21 Bjork-Shiley disc valve. The devices of Yu and Taenaka lack simplicity in design and fabrication, and are at the limits of down-sizing due to their valve constraints. Such limitations do not exist for the invention as claimed herein.
Potential applications of the proposed device include the approximately 20,000 pediatric cardiac surgery cases performed in the United States annually. Initial application will be in cases involving post-cardiotomy support subsequent to the correction of congenital heart defect or in patients with cardiopulmonary compromise secondary to myocarditis and/or cardiomyopathy.